Are you trying to print a document or take a screen shot? The Prnt Scrn (Print Screen) button is used to copy an image of what you see on your monitor to the system's clipboard which you can then paste that image to some image editing software such as Windows Paint.
If you're trying to print a document, press Ctrl + P or go to File>Print. In here a new window should appear similar to the one in this
screen-shot (Window probably won't look exactly the same). What you need to do is select the option to print pages instead of all. This is shown in this
Now, if you want to, for instance, print page 5 of a document. You would fill in the from - to boxes of the, "Pages" range with the number 5. This rule applies to any page or range of pages.
As for the F keys, that depends on what program you're using. Some applications will utilize the keys to perform certain functions while other applications won't use the keys. For instance, in most applications pressing F1 brings up a help dialog. However the main use for the F keys is during the startup of the computer. The F keys will be used to enter the system BIOS and to install Operating Systems.
It is fairly difficult for me to explain the main functions of the F keys because they are just as the letter keys, they have functions yet they have to be defined by an application before the keys have any use. Therefore there are no set rules as to what the F keys can do or will do as it changes from application to application.
I hope that answers your questions. If not, let me know and I'll try to give you the answer you were seeking.